I developed a prototype outside of work using my own materials and time. The end product would be very useful in my field of work so I showed it to my boss, who now wants me to finish development in house using his materials and paying me for my time. Will I automatically lose the rights (barring a written contract) to my invention if I begin this work, despite already creating a working prototype? Located in CO, USA.
Assuming your invention is completely unrelated to your work, I would first negotiate with your employer before allowing them any access. If fact I wouldn't have disclosed it to your boss without first signing a confidentiality agreement. I'm not a lawyer and I don't know what your employment contract contains. That said, I think if you develop something on your own time with your own resources not using any knowledge related to your employment, it should be your own property. You may or may not want to license it to your employer.
I think it would be a very good idea to consult with an intellectual property attorney before taking any additional steps. It might especially be useful to file a provisional patent application. You want to maximize the value of the invention. That might be done by licensing to your company. That might or might not be an exclusive license. In any case a lawyer can help you protect your invention and realize its full value.